Montenegro was formally invited to join the alliance in 2015 and despite awaiting its formal acceptance, participated as a full member during the NATO leaders meeting in Brussels last month.

Monday's event comes on the heels of President Donald Trump's controversial appearance at that meeting of world leaders, where he lectured fellow heads of state over their defense spending and failed to reaffirm US commitment to Article 5 of the treaty -- a key provision that outlines the principle of collective defense.

While Trump proclaimed the trip a "great success for America," European leaders have been critical of Trump's remarks, as well as his decision to withdraw from the Paris climate accord, seeing the President's actions as signs the US is ceding its global leadership role in favor of an inward turn.

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On Monday, however, acting Assistant Secretary of State for European and Eurasian Affairs John Heffern tried to strike a different tone in his remarks to the crowd of ambassadors and foreign officials, directly referencing the importance of Article 5 and collective defense.